In a vehicle electronic key system, an activation means (a switch) is installed in a door handle and/or a trunk lid of a four-wheeled vehicle or the like. When a user operates (activates) the activation means, the communication between the activation means and a portable device is started, the verification between an ID transmitted from the portable device and an ID registered in a vehicle-mounted device is performed, and a door lock or the like is released at the stage when the result of the verification showing the correspondence between the IDs is obtained. See, for example, Japanese Patent Laid-Open No. 2001-349117, paragraphs [0026] to [0028].
Japanese Patent Laid-Open No. 2001-349117 discloses an activation means (a switch) that is installed in an ignition knob. Once the user operates the ignition knob after getting in the four-wheeled vehicle, the communication between the activation means and the portable device is performed again to carry out the ID verification for the purpose of permitting the engine to be started. In addition, Japanese Patent Laid-Open No. 2001-349117 describes an engine that is started at the stage when the result of the verification showing the correspondence between the IDs is obtained.
In other words, with regard to the conventional vehicle electronic key systems for four-wheeled vehicles, the presence of an authorized portable device is a necessary requirement for unlocking a door and starting the engine.
As a vehicle electronic key system implemented in a motorcycle, a system has been proposed which discloses that the system generates a random number on the basis of the time (start time data) from when an immobiliser is turned on to the time when an engine is started, generates an authentication key and an authentication code by encrypting unique information of a key on the basis of the random number, and performs authentication with the use of the authentication key and the authentication code. The system is designed to prevent theft. See the abstract of Japanese Patent Laid-Open No. 2001-12123.
As a vehicle electronic key system implemented in a motorcycle or the like, a system has been proposed, which reduces the probability of loss of a portable device by illuminating a warning light if a user dropped the portable device while a vehicle is stopped, or running. See the abstract of Japanese Patent Laid-Open No. 2004-114860.
With regard to vehicle electronic key systems, a portable device may be activated by a request signal which is the authorized radio signal from a vehicle, and performs an authentication operation. However, it is possible that the portable device is activated successively by radio noise other than the authorized radio signal.
If the activation is performed by continuous radio noise, it comes to be difficult to start the authentication based on the request signal. As a result, it is possible to conceive that it can take a long time to unlock a main switch knob or the like of the vehicle to switch to an engine starting position.
Broadcasting facilities, power plants, power supply devices, unnecessary radiation emitted from cables connected thereto, and the like are noise sources that generate radio noise continuously.